1. Field of the Invention
The present invention relates to an image processing apparatus, and more specifically, to an image processing apparatus capable of an excellent reproduction of an intermediate gradation.
2. Related Background Art
Conventionally, since an image formation apparatus such as a laser beam printer or the like has difficulty expressing a gradating image, a method of reproducing a pseudo-intermediate-gradation referred to as an area gradation has been used.
This intermediate gradation reproduction method includes:
(1) a method of reproducing an intermediate gradation referred to as a so-called pulse width modulation (PWM) system, wherein, typically, digital input image data is subjected to a D/A conversion, and the thus D/A converted analog signal is compared with a reference triangular wave signal to thereby perform an ON/OFF control of a laser beam, as disclosed in U.S. Pat. No. 4,800,422; and
(2) a method of effecting a binary coding processing referred to as a dither method, a density pattern technique or the like, wherein when a printer has a sufficiently high record resolution by itself, digital input image data is subjected to a binary coding processing with a clock having a frequency higher than that of a pixel clock of the input image data to thereby reproduce an intermediate gradation, other methods may also be used.
Nevertheless, the above pulse width modulation method is disadvantageous in that an offset of the reference triangular signal and a gain of a D/A converter must be adjusted, which adjustment is time-consuming.
The above dither method and density pattern technique are also disadvantageous in that when the number of gradations is to be increased, the quality of an image is greatly degraded unless the printer has an increased record resolution. Consequently, although a clock frequency used for the binary coding processing must be increased with an increased circuit cost, the number of gradations cannot be increased too much because the operating speed of the devices is limited. In addition, the number of lines capable of being produced (resolution) must be reduced in order to realize a high gradation while restricting a circuit cost and operating speed.
Further, U.S. Pat. No. 4,926,248 discloses a method of expressing a gradating image by generating a PWM signal through digital processing. In this method, however, a problem arises in that cost is increased by the use of an expensive circuit element because a high frequency clock is required to generate a triangular wave which changes stepwise, and in addition a countermeasure for unnecessary radiation is needed. Furthermore, although U.S. Pat. No. 4,370,667 discloses a technology relating to the generation of a PWM signal, this technology has the same problem as that of U.S. Pat. No. 4,926,248.